Mycorrhizae confer aluminum resistance to tulippoplar seedlings

The biomass of mycorrhizal tulip-poplar seedlings was greater than that of non-mycorrhizal seedlings across all Al treatments. Shoot and root mass were three- to five-fold greater in mycorrhizal seedlings than in non-mycorrhizal seedlings (P < 0.01) (Fig. 2). Leaf area and mass followed a similar pattern, and were, respectively, 5 and 6.5 times greater in mycorrhizal plants than in non-mycorrhizal plants (Table 1). Mycorrhizal plants allocated proportionally less carbon to roots than non-mycorrhizal plants, as demonstrated by a root/shoot ratios of 0.66 for mycorrhizal plants compared with a value of 0.77 for non-mycorrhizal plants, at the 0 (mu)M Al level (Table 1). The biomass of non-mycorrhizal, but not mycorrhizal, tulip-poplar seedlings was significantly less when exposed to Al in solution (Fig. 2). Leaf area and mass of non-mycorrhizal seedlings exposed to Al were up to 52% less than the values obtained for the non-mycorrhizal controls, whereas there were no differences in leaf area and mass of mycorrhizal seedlings exposed to Al treatments and their controls (Table 1). Aluminum also influenced the allocation of carbon between roots and shoots in nonmycorrhizal seedlings, having a greater negative effect on shoot biomass (Fig. 2, Table 1). These relationships between biomass and tissue elemental concentrations suggest that Al interfered with the capacity of non-mycorrhizal seedlings to acquire sufficient P to translocate to foliage and also affected the entry of Ca and Mg into roots, which led to reductions in biomass. These limitations could be due to Al-P precipitation reactions in the root or rhizosphere (Clarkson 1967; Cumming et al. 1986) and Al - divalent cation interactions in the Donnan free space (Shortle and Smith 1988; Cronan 1991), as noted above. In mycorrhizal seedlings, the lack of correlation between plant biomass and these nutrient variables may reflect the effective P uptake systems of mycorrhizal fungi and the ability of fungi to overcome the interactions between Al and Ca, Mg, and P. While the mechanisms for these responses are not known, ion acquisition under Al exposure may be facilitated by the production of metal chelating compounds by mycorrhizal fungi, which reduce the concentration of labile Al in the rhizosphere, as noted in Fig. 1, and by subsequent interactions between Al and these ions in the rhizosphere and root (Cumming and Weinstein 1990; Huang et al. 1996; Cumming et al. 2001). Responses of mycorrhizal seedlings to